Probing the Radical and Base Dual Properties of Peptide Sulfinyl Radicals via Mass Spectrometry

Heteroatom-centered radicals are known to play critical roles in atmospheric chemistry, organic synthesis, and biology. While most studies have focused on the radical reactivity such as hydrogen abstraction, the base properties of heteroatom-centered radicals have long been overlooked, despite the profound consequences, such as their ability to participate in hydrogen-bonding networks. In this study, we use the sulfinyl radical (−SO^•) as a model to show that the dual properties of heteroatom-centered radicals, that is, their ability to function as a radical and a base, can coexist in peptides and be differentiated by examining the loss of hydrosulfinyl radical (SOH) upon unimolecular dissociation of the peptide sulfinyl radical ions in the gas phase. The loss of SOH can result from two channels; one involves hydrogen atom abstraction, which reflects the radical property; the other is initiated by proton transfer to the sulfinyl radical, manifesting its base property. Tuning of the two properties of peptide sulfinyl radicals can be achieved by varying the chemical properties of the neighboring functional groups, which demonstrates the influence of the local chemical environment on the behavior of the radical species. The experimental approach established in this study to probe the dual chemical property of the peptide sulfinyl radical can be potentially applied to studying other types of heteroatom-centered radical species of biological significance.